Automatic lubrication systems are known that include mechanical dosing and pumping components. These components may be configured to provide a fixed dose of lubricant (grease or oil or other suitable lubricant, referred to hereinafter generally as “grease”) at predetermined intervals. The intervals and doses are conventionally set by manually adjusting each of the individual dosing devices. A lubrication system that provides a greater degree of control and more automated control over lubricant dosing is disclosed in U.S. Patent Application Publication No. 2013/0015019 A1 for an “Advanced Lubrication System,” which disclosure is hereby incorporated by reference.
An example of a mechanical system that requires lubrication is a bearing assembly encased in a bearing housing. These systems include a housing containing the bearing, and the housing has a free volume, that is, a volume potentially finable with lubricant when lubricant is pumped into the housing through a lubrication inlet. A shaft extends through an opening in the housing, and a seal is provide between the shaft and the housing, either on the shaft or on the housing. Seals may either be fixed to a housing and have a radially inwardly extending lip configured to ride on or engage a rotatable shaft or may be fixed to a shaft and have a radially outwardly extending lip configured to ride on or engage the opening in the housing. The following discussion will generally discuss seals mounted on a housing but applies equally to shaft-mounted seals.
Bearing housings also generally include one or more pressure relief valves (purge valves), which may comprise, for example, spring-biased check valves. When the pressure in the housing reaches a certain level, 5 psi, for example, the relief valve opens and allows lubricant to escape through the relief valve until the pressure drops below the certain level. However, bearing pressure relief valves are notoriously unreliable and prone to frequent failure. When these valves fail, lubricant may leak uncontrollably from the housing and produce an under-lubrication condition for the bearing which could rapidly lead to bearing damage or failure if not quickly addressed.
Conventional lubrication systems are generally configured to provide lubricant based on the needs of the bearing or component being lubricated. When a determination is made that the bearing requires additional lubricant, a pump may be actuated to pump additional lubricant into the bearing housing. The housing is generally completely full of lubricant and pressurized to some extent 2-5 psi, for example. The pressure in the housing while the bearing is operating will generally be lower than the pressure at which the pressure relief valve opens. Adding a sufficient lubricant to the bearing housing can increase the pressure to a level greater than 5 psi and cause lubricant to be expelled through the pressure relief valve. Adding lubricant intermittently or periodically to the bearing housing without raising the pressure above 5 psi helps ensure that adequate lubrication is provided for the bearing at all times.
Bearing housings are used in many different environments. In the mining field and various industrial fields, bearing housings are exposed to dust, dirt and other contaminants which will damage a bearing if they are able to enter the bearing housing. Exclusion seals are particularly useful for providing a seal between a shaft and a bearing in such environments. Exclusion seals are configured to keep dirt and other contaminants out of a bearing housing. When exclusion seals are new, they accomplish this exclusion of dirt by maintaining a tight seal between the shaft and the housing. The interface between the shaft and seal is so tight that dirt and other contaminants impinging on the shaft/seal interface are unable to fit between the shaft and seal. When exclusion seals wear, they provide a less secure seal and are less able to physically block contaminants from entering the bearing housing. However, beneficially, worn exclusion seals allow a small amount of grease to leak from the bearing housing, and this outward leakage of grease helps push contaminants that accumulate or impinge against the shaft/seal interface away from the interior of the bearing housing.
To maximize an exclusion seal's ability to exclude contaminants, it would generally be desirable to maintain as tight a seal as possible between the seal and the shaft. However, the friction produced at the shaft/seal interface and thus the frictional drag on the shaft is related to the pressure with which the seal engages the shaft. High pressure also causes the seal to wear faster that a seal that engages the shaft with a light pressure. Therefore, a balance must be struck between keeping the seal as tight as possible to keep contaminants out of the bearing housing and using a loose seal to minimize the energy usage required to drive a shaft in physical engagement with the seal.
It would be desirable to provide a bearing system that avoids the problems of conventional, unreliable, pressure relief valves and that also ensures a high degree of contaminant exclusion by an exclusion seal, even as the seal wears.